1. Field of the Invention
The present invention relates to a robot system and its control method and is preferably applied to, for example, an automatically-walking system.
2. Description of the Related Art
A conventional automatically-walk robot includes the multiple-leg type by supporting the load of the robot by a plurality of legs. The pantograph type and the so-called joint type constituted by connecting joints by links are considered as leg mechanisms. For example, the case of the "leg mechanism of a walk robot" disclosed in Japanese Patent Publication No. 4-74150, the pantograph type is adopted as a leg mechanism and a mechanical system for efficiently driving the front end of the leg mechanism is described.
The insect-type robot 1 in FIGS. 35A and 35B has six joint-type leg mechanisms 2 to 7.
Each of the leg mechanisms 2 to 7 comprises two leg links 8 and two actuator-provided joints (respectively shown by a hatched circle in FIGS. 35A and 35B) 9. The insect-type robot 1 stands up by driving the horizontal leg mechanisms 2 to 7 by the actuator-provided joints 9 arranged at the roots of the leg mechanisms 2 to 7.
As a walk robot, a type is considered which has an arm not used for walking and works by moving the arm. A human-type robot or a bird-type robot walks with a two-leg mechanism and it is assumed that the robot works by an arm mechanism provided separately from the leg mechanism.
For example, Osuga et al. discuss the control of bending and stretching of knees of a bird-type two-leg robot in "Nonlinear control of two-leg walk robot," Twenty-third Control Theory Symposium, 1994, pp. 313-318. It is assumed that the two-leg walk robot walks with a two-leg mechanism and maintains a standing attitude by the two-leg mechanism when working by using two hands.
However, the two-leg walk robot requires complex control when making the robot only walk with a two-leg mechanism. Therefore, when working by using hands, the two-leg walk robot has a problem that the control for stably keeping a standing attitude is complicated compared to the case of walking.
However, Arai el al. propose a "rim mechanism" constituted by unifying an arm mechanism and a leg mechanism into one body without mounting a manipulator on the leg mechanism in "Study of unification of leg and arm," Eleventh collection of scientific lectures of Robotics of Society of Japan, 1993, pp. 629-630. This robot having a rim mechanism uses his arms also for walking. When the robot works, it performs control so as to stabilize the attitude by keeping the balance with a plurality of leg mechanism links not used for working.
However, to stabilize the attitude by keeping the balance at the time of working, there is a problem that control of leg mechanism links is complicated.